1. Field of the Invention
The invention relates to optical apparatus and methods for measuring visibility through the atmosphere and is more particularly concerned with the measurement of the extinction coefficient of an atmospheric backscattering medium by techniques from which reliable measures can be achieved over a long atmospheric path through a variable density atmosphere.
2. Description of the Prior Art
Prior art concepts of interest with respect to the present invention are those of two United States patents:
U.s. pat. No. 3,519,354 -- R. T. Brown, R. F. Hazel, H. Laudon--"System for Measuring Extinction Coefficients in the Atmosphere Utilizing Backscattered Signals", issued July 7, 1970, and PA1 U.s. pat. No. 3,782,824 -- A. P. Stoliar, R. T. Brown--"Apparatus and Method for Measuring Extinction Coefficient of an Atmospheric Scattering Medium", issued Jan. 1, 1974;
Both patents are assigned to Sperry Rand Corporation.
The Brown et al patent concerns a single-ended system for the optical measurement of meteorological visual range under atmospheric conditions such as those tending to restrict visibility along aircraft landing runways, the measurement being made in terms of certain characteristics of backscattered pulsed optical signals. Brown et al employ a monochromatic, slightly divergent light beam radiated by a pulse transmitting laser. A cooperating optical receiver having a similarly divergent field of view is spaced laterally from the optical transmitter so that its field of view overlaps most of the length of the radiated beam, the overlapped relation necessarily beginning at a predetermined distance from the laser emitting face. The backscattered light pulse energy is analyzed to compute an average atmospheric extinction coefficient associated with the medium within the field of view of the receiver.
In the improvement of the Stoliar et al patent, the side-by-side relation of the transmitter and receiver patterns is no longer used, and the apparatus is improved to eliminate the need for highly precise calibration and optical alignment and to assure more accurate measurements irrespective of the absorption, backscatter, composition, and homogeneity characteristics of the backscattering medium. The Stoliar et al device is a single-ended system that demonstrates the advantage of not being restricted to making measurements in a homogeneous medium, in that the medium no longer has to be such that its extinction coefficient is substantially constant over a comparatively long range.
The Stoliar et al device employs a monochromatic optical transmitter for transmitting a series of optical pulses in a slightly diverging beam along a common propagation path in the scattering medium whose extinction coefficient is to be measured. A receiver sharing the common propagation path and proximate the optical transmitter has a field of view coaxially aligned with that propagation path so as to overlap substantially the full length of the transmitter irradiation field. Accordingly, the receiver readily detects optical pulse energy backscattered from the scattering medium. First and second integrators, for example, receive first and second successively returned light pulses and the logarithm of the ratio of the amplitudes of these integrated signals is derived to determine the extinction coefficient. The arrangement of the Stoliar et al patent is a significant advance in the art because it minimizes parallax effects found in the Brown et al system; however, it does not fully eliminate the problem of nonoverlapping beams for scattering at very short ranges and other problems causing poor signal-to-noise characteristics tend to arise when a compromise configuration reducing the parallax problem is attempted, as will be further discussed.